Several recent developments have occurred in electrophotographic photoreceptor technology. New photoreceptors have a vapor deposited photosensitive layer consisting principally of amorphous silicon. This trend has developed because amorphous silicon photoreceptors have an increased life expectancy over conventional electrophotographic photoreceptors. The application of amorphous silicon to electrophotographic photoreceptors results in more electrically stable repeat characteristics, increased hardness, greater thermal stability, and a longer life expectancy.
In the past, a variety of amorphous silicon based electrophotographic photoreceptors have been proposed, for example, JP-A-54-78l35 and JP-A-54-8634l (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
Of these, a superior embodiment comprises an amorphous silicon based electrophotographic photoreceptor with functionally separated photosensitive layers. These layers include a charge generating layer wherein charge carriers are generated upon irradiation with light, and a charge transporting layer into which the charge carriers can be injected and transferred efficiently. The photosensitive layers include amorphous silicon films formed by glow-discharging a mixed gas comprising a gas of silane compounds such as silane or disilane, a gas containing carbon, oxygen or nitrogen, and a gas containing very small amounts of group III or group V elements. The amorphous silicon based electrophotographic photoreceptor just described was proposed in JP-A-62-9355.
In general, electrophotographic photoreceptors in which the charge transporting layer and the charge generating layer are functionally separated display charging properties which are affected by the characteristics of the charge transporting layer which comprises the largest portion of the photosensitive layer. The chargeability of an electrophotographic photoreceptor including a hydrogenated amorphous silicon film obtained by means of the glow discharge of the silane compounds mentioned above is about 30 V/.mu.m or less. This figure is still inadequate. Further, the dark decay rate, which differs according to the conditions of use, in general, is very high, being at least some 20%/second. Consequently, electrophotographic photoreceptors which have an amorphous silicon based charge transporting layer of this type are limited to use in comparatively high speed systems. Otherwise, a specific development system is required since an adequate charge potential cannot be obtained. The thickness of the charge transporting layer can be increased in order to increase the charge potential, but this not only increases the time required to produce the film but also increases the probability of defects being produced while forming the film.
The prior art reference JP-A-63-6305l (corresponding to U.S. Pat. application Ser. No. 93,285) taught the use of an aluminum oxide film to function as the charge transporting layer.
However, when an aluminum oxide film is formed by methods such as ion plating or electron beam vapor deposition, cracks are generated on the film depending on various formation conditions, and transparency of the film deteriorates. Accordingly, when such a cracked aluminum oxide film is used as the charge transporting layer, the obtained electrophotographic photoreceptor displays unstable electric characteristics, and produces defective images.
The present invention was developed in view of problems of the conventional techniques such as those discussed above.